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AlInAs/AlGaAs quantum dots (QDs) have emerged as excellent emitters across the visible spectral range, showcasing highly tunable electronic properties through variations in composition and size. This versatility allows for diverse band alignments within the same system. In this study, we present compelling evidence for the coexistence of type-I indirect and direct emissions from QDs, supported by comprehensive analyses of their photoluminescence responses to excitation power, temperature, and time, along with band structure calculations. The high-density QD system exhibits signs of lateral coupling, facilitated by carrier transfer between dots, modulated by energy barriers and recombination times. Additionally, we can unequivocally prove that the QDs act as carrier reservoirs that progressively feed optically active states in the bulk GaAs at low temperaturesan attractive prospect for hot-carrier photovoltaics. Above certain temperatures, the bulk system reverts to the anticipated predominantly radiative recombination dynamics. Our theoretical framework, accounting for the coexistence of QD specimens with varying recombination times, successfully elucidates the optical response at different temperatures, emphasizing the pivotal role of QD excitation in enhancing the effective lifetime of carriers in the bulk. [ABSTRACT FROM AUTHOR]

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AbstractInAs/GaAs heterostructures have been simultaneously grown by molecular beam epitaxy on GaAs (100), GaAs (100) with a 2° misorientation angle towards [01−1], and GaAs (n11)B (n = 9, 7, 5) substrates. While the substrate misorientation angle increased from 0° to 15.8°, a clear evolution from quantum dots to quantum well was evident by the surface morphology, the photoluminescence, and the time-resolved photoluminescence, respectively. This evolution revealed an increased critical thickness and a delayed formation of InAs quantum dots as the surface orientation departed from GaAs (100), which was explained by the thermal-equilibrium model due to the less efficient of strain relaxation on misoriented substrate surfaces.

AbstractThe evolution of InAs quantum dot (QD) formation is studied on GaAs ring-like nanostructures fabricated by droplet homo-epitaxy. This growth mode, exclusively performed by a hybrid approach of droplet homo-epitaxy and Stransky-Krastanor (S-K) based QD self-assembly, enables one to form new QD morphologies that may find use in optoelectronic applications. Increased deposition of InAs on the GaAs ring first produced a QD in the hole followed by QDs around the GaAs ring and on the GaAs (100) surface. This behavior indicates that the QDs prefer to nucleate at locations of high monolayer (ML) step density.

The identification of cross transitions from the wetting layer (WL) valence band to the electron ground states of self-assembled InGaAs quantum dots (QDs) in the photoluminescence (PL) of QD ensemble are presented. The QD PL dependences on temperature and excitation density might be due to interdot carrier transfer through the continuum states related to the wetting layer morphology and to phonon-assisted processes.

Time-resolved photoluminescence measurements is carried out for InAs quantum dots (QDs) coupled through a GaAs barrier to an In0.3Ga0.7As quantum well (QW). A rate equation model is developed to describe the observed behavior through carrier capture from QW to QD states in the QD:QW structure.

Time-resolved photoluminescence (PL) measurements is conducted for the InGaAs quantum dots (QD) grown on (001) and (311) oriented GaAs substrates. Strong dependence of the decay time on the excitation density observed for the QD ground-state PL is consistent with a presented theoretical model.

Temperature-dependent photoluminescence is investigated in bilayer InAs/GaS quantum dot structures. Results reveal that interlayer coupling, leading to the formation of asymmetric quantum dot pairs, strengthens the high-temperatures photoluminescence and strongly influences carrier relaxation channels.

Carrier transfer was studied in self assembled coupled InAs/GaAs quantum dots.

Photoelectric properties of laterally correlated multilayer InGaAs/GaAs quantum dots (QDs) heterostructures are studied. The response of the photocurrent to increasing excitation intensity is found to be nonlinear and varying with excitation energy. The structures are photosensitive in a wide range of photon energies above 0.6 eV. The spectral dependence of the photoconductivity (PC) is caused by strong interaction between the bulk GaAs and the lower energy states of the wetting layer, the QDs, as well as the defect states in the GaAs band gap. In particular, a mechanism for the participation of deep electron trap levels in the photocurrent is clarified. These structures also demonstrate a high sensitivity of up to 10A/W at low excitation intensities. However, at higher excitation intensities, the sensitivity reduces exhibiting a strong spectral dependence at the same time. The observed sublinear PC dependence on excitation power results from a direct electron-hole recombination both in the QDs and in GaAs host. The solution of rate equations included the contributions of QD ground and exited states, bulk GaAs states and the states of defects within the GaAs bandgap describes well the experimental data. [ABSTRACT FROM AUTHOR]

The interplay between structural properties and charge transfer in self-assembled quantum ring (QR) chains grown by molecular beam epitaxy on top of an InGaAs/GaAs quantum dot (QD) superlattice template is analyzed and characterized. The QDs and QRs are vertically stacked and laterally coupled as well as aligned within each layer due to the strain field distributions that governs the ordering. The strong interdot coupling influences the carrier transfer both along as well as between chains in the ring layer and dot template structures. A qualitative contrast between different dynamic models has been developed. By combining temperature and excitation intensity effects, the tuning of the photoluminescence gain for either the QR or the QD mode is attained. The information obtained here about relaxation parameters, energy scheme, interlayer and interdot coupling resulting in creation of 1D structures is very important for the usage of such specific QR-QD systems for applied purposes such as lasing, detection, and energy-harvesting technology of future solar panels. [ABSTRACT FROM AUTHOR]

Purpose. To analyze the current state of innovation and environmental projects, to develop management strategies and algorithms for project environmental and economic management in subsoil use, to propose a mathematical model of the system of perception and support of solutions for effective environmental and economic management. Methodology. Different methods of cognition were used for scientific research – both general and special. Analytical study on the state and realities of innovation in Ukraine in general and in particular in the mining industry, identification of trends in world practice were conducted based on using the methods of content analysis, quantitative and qualitative comparison. Methods of scientific abstraction and systematization were used to develop strategies and basic algorithm of project environmental and economic management in subsoil use. Mathematical methods were used to develop a mathematical model and coordinate it with the developed algorithm of project environmental and economic management for subsequent use in the system of perception and support of decisions. Findings. A systematic approach to the definition of management strategies and an algorithm of project environmental and economic management for the implementation of effective economic and environmental management of mining enterprises is proposed. A mathematical model designed for use in the system of perception and support of decisions for effective environmental and economic management has been developed. An analytical study on the state of innovation in Ukraine in general and in particular in the extractive industry has been conducted. This provided an opportunity to identify trends in the dynamics of change regarding the introduction of innovations and identify the causes of existing trends. Originality. The study identified threatening trends in the practice of introducing innovation. To solve production and scientific problems in the presence of significant challenges, new tools for finding optimal solutions in the environmental and economic management of subsoil use were proposed. Using a systematic approach, a mechanism for selecting strategies, an algorithm of project environmental and economic management and an original mathematical model have been proposed. Practical value. The results of the study can be used both to create new tools for information support of environmental and economic management, and by scientists and practitioners to develop effective solutions for the management and implementation of innovative projects in subsoil use. [ABSTRACT FROM AUTHOR]

Abstract A method to determine the effects of the geometry and lateral ordering on the electronic properties of an array of one-dimensional self-assembled quantum dots is discussed. A model that takes into account the valence-band anisotropic effective masses and strain effects must be used to describe the behavior of the photoluminescence emission, proposed as a clean tool for the characterization of dot anisotropy and/or inter-dot coupling. Under special growth conditions, such as substrate temperature and Arsenic background, 1D chains of In0.4Ga0.6 As quantum dots were grown by molecular beam epitaxy. Grazing-incidence X-ray diffraction measurements directly evidence the strong strain anisotropy due to the formation of quantum dot chains, probed by polarization-resolved low-temperature photoluminescence. The results are in fair good agreement with the proposed model.

Here, we demonstrate X-ray fitting through kinematical simulations of the intensity profiles of symmetric reflections for epitaxial compositionally graded layers of AlGaN grown by molecular beam epitaxy pseudomorphically on [0001]-oriented GaN substrates. These detailed simulations depict obvious differences between changes in thickness, maximum concentration, and concentration profile of the graded layers. Through comparison of these simulations with as-grown samples, we can reliably determine these parameters, most important of which are the profiles of the concentration and strain which determine much of the electrical properties of the film. In addition to learning about these parameters for the characterization of thin film properties, these fitting techniques create opportunities to calibrate growth rates and control composition profiles of AlGaN layers with a single growth rather than multiple growths as has been done traditionally. [ABSTRACT FROM AUTHOR]

The in-plane photoconductivity and photoluminescence are investigated in quantum dot-chain InGaAs/GaAs heterostructures. Different photoconductivity transients resulting from spectrally selecting photoexcitation of InGaAs QDs, GaAs spacers, or EL2 centers were observed. Persistent photoconductivity was observed at 80K after excitation of electron-hole pairs due to interband transitions in both the InGaAs QDs and the GaAs matrix. Giant optically induced quenching of in-plane conductivity driven by recharging of EL2 centers is observed in the spectral range from 0.83 eV to 1.0 eV. Conductivity loss under photoexcitation is discussed in terms of carrier localization by analogy with carrier distribution in disordered media. [ABSTRACT FROM AUTHOR]

Strong optical anisotropy is observed in the emission from a GaAs1–xBix (x~0.04) quantum well grown by low temperature molecular beam epitaxy on (001) GaAs by means of low temperature magneto-photoluminescence (MPL) taken at 2K in Faraday geometry for magnetic fields, B, up to 10 T. A significant diamagnetic shift (~2.5meV) develops for magnetic fields above ~8T, which is accompanied by a narrowing of the emission bandwidth and a substantial increase in the difference between the integrated intensities of the r+ and r– polarizations in the MPL spectra. This, along with a peculiar spectral dependence of the polarization degree which evolves with increasing magnetic field, is interpreted in terms of bound and free magneto excitons in the system where Bi-related levels become hybridized to different extents with the valence and conduction bands of the GaAs host material. [ABSTRACT FROM AUTHOR]

Detailed experimental and theoretical studies of lateral electron transport in a system of quantum dot chains demonstrate the complicated character of the conductance within the chain structure due to the interaction of conduction channels with different dimensionalities. The one-dimensional character of states in the wetting layer results in an anisotropic mobility, while the presence of the zero-dimensional states of the quantum dots leads to enhanced hopping conductance, which affects the low-temperature mobility and demonstrates an anisotropy in the conductance. These phenomena were probed by considering a one-dimensional model of hopping along with band filling effects. Differences between the model and the experimental results indicate that this system does not obey the simple one-dimensional Mott's law of hopping and deserves further experimental and theoretical considerations. [ABSTRACT FROM AUTHOR]

Excitonic dynamics in a hybrid dot-well system composed of InAs quantum dots (QDs) and an InGaAs quantum well (QW) is studied by means of femtosecond pump-probe reflection and continuous wave (cw) photoluminescence (PL) spectroscopy. The system is engineered to bring the QW ground exciton state into resonance with the third QD excited state. The resonant tunneling rate is varied by changing the effective barrier thickness between the QD and QW layers. This strongly affects the exciton dynamics in these hybrid structures as compared to isolated QW or QD systems. Optically measured decay times of the coupled system demonstrate dramatically different response to temperature change depending on the strength of the resonant tunneling or coupling strength. This reflects a competition between purely quantum mechanical and thermodynamical processes. [ABSTRACT FROM AUTHOR]

A set of high quality single quantum well samples of GaAs1-xBix with bismuth concentrations not exceeding 6% and well widths ranging from 7.5 to 13 nm grown by molecular beam epitaxy on a GaAs substrate at low temperature is studied by means of photoluminescence (PL). It is shown that the PL line shape changes when the exciton reduced mass behavior changes from an anomalous increase (x < 5%) to a conventional decrease (x > 5%). Strongly non-monotonous PL bandwidth dependence on the excitation intensity is revealed and interpreted in terms of optically unresolved contributions from the saturable emission of bound free excitons. [ABSTRACT FROM AUTHOR]

Change of the photoluminescence (PL) polarization is studied by changing the excitation intensity and temperature for aligned In(Ga)As quantum dot (QD) structures with varying inter-dot distances grown by molecular beam epitaxy on semi-insulating GaAs (100) substrates. An unusual increase of the polarization ratio is observed by increasing the temperature and/or excitation intensity throughout a low temperature (T < 70 K) and low intensity (Iex < 1 W/cm2) range. This increase as well as the general behavior of the polarized PL are the results of the exciton dynamics and the peculiarities of the system morphology. They are due to the varying inter-dot distances which change the system from zero-dimensional comprised of isolated QDs to one-dimensional comprised of wire-like structures. [ABSTRACT FROM AUTHOR]

Using molecular beam epitaxy, we prepared an anisotropic media consisting of InGaAs quantum wires epitaxially grown on GaAs (311)A. Anisotropy is observed in the lateral conductivity and photoluminescence polarization. However, an isotropic Hall effect is observed in the same samples. We show that the Hall effect in this anisotropic heterostructure remains isotropic regardless of the change of the doping in GaAs barriers and regardless of the InGaAs coverage, whereas the conductivity anisotropy experiences a strong change under these actions. In addition, we observed an anomalous increase in carrier density, 'freeze-in,' at low temperatures. In order to explain this, we generalized the theory of Look [D. C. Look, Phys. Rev B 42, 3578 (1990)] by considering the low field magneto-transport in anisotropic media. This theory confirms that the Hall constant remains isotropic in anisotropic semiconductor heterostructures, agreeing with our experiment and explains the anomalous behavior of carriers as a result of multi-band conductivity. [ABSTRACT FROM AUTHOR]